1. Field of the Invention
Embodiments of the present invention generally relate to semiconductor processing including forming a silicon oxynitride film. More specifically, the present invention relates to a method for forming a silicon oxynitride film using plasma nitridation and a two step annealing process.
2. Description of the Related Art
As device geometries of integrated circuits and transistors have decreased, the gate drive current required by the transistors have increased. It is known that a gate drive current of a transistor increases as its gate capacitance increases, and the gate capacitance of a transistor is equal to k*A/d, where k is the dielectric constant of the gate dielectric (which is usually silicon oxide), d is the dielectric thickness, and A is the gate contact area. Thus, decreasing the dielectric thickness and increasing the dielectric constant of the gate dielectric are two ways of increasing the gate capacitance and the drive current.
Attempts have been made to reduce the thickness of dielectrics, such as reducing the thickness of silicon dioxide (SiO2) dielectrics to below 20 Å. However, the use of SiO2 dielectrics with thicknesses below 20 Å often results in undesirable performance and decreased durability. For example, boron from a boron doped electrode can penetrate through a thin SiO2 dielectric into the underlying silicon substrate. In addition, there is typically an undesirable increase in the gate leakage current, i.e., tunneling current, that in turn increases the amount of power consumed by the gate. Thin SiO2 gate dielectrics may also be susceptible to negative-channel metal-oxide semiconductor (NMOS) hot carrier degradation and to positive-channel metal-oxide semiconductor (PMOS) negative bias temperature instability (NBTI).
Nitridation of the SiO2 layer has been employed as a way to reduce the thickness of the SiO2 dielectric layer to below 20 Å. Plasma nitridation is used to incorporate nitrogen into the gate oxide. Nitridation provides high nitrogen concentration at the electrode/oxide interface. The high nitrogen concentration at the interface prevents boron penetration into the gate oxide. The bulk gate oxide dielectric, on the other hand, is lightly doped with nitrogen during the plasma nitridation process. The low nitrogen concentration of the bulk results in a film with lower equivalent oxide thickness (EOT) than the starting oxide, thereby reducing gate leakage. It is desirable to provide a dielectric with EOT<12 Å.
Annealing silicon oxynitride after nitridation improves channel mobility but at the expense of EOT increases as observed by performing peak transconductance measurements of traditionally annealed silicon oxynitride films. Channel mobility is degraded more at lower EOT thickness than at higher EOT thickness. Also, higher EOT decreases drive current of traditionally annealed silicon oxynitride films. Thus, an annealing process that provides films with desirable channel mobility, drive current, and EOT is needed.